BACKGROUND: Obesity is the single most significant nutrition-related health issue of the new millennium. Several "medical experts" have designed and promoted weight loss diets that dramatically differ from one another, and from the USDA Dietary Guidelines. These diets have gained surprisingly widespread and persistent popularity among Americans, despite a lack of evidence supporting their claims. OBJECTIVE: To conduct a preliminary/feasibility study for an intended full-scale study that will examine health outcomes (e.g. benefits, risks, success) of three popular weight loss diets with very different macronutrient composition, and compare them to a "usual care" dietary approach to weigh loss represented by the macronutrient distribution of the USDA Food Pyramid. We will investigate behavioral and physiological factors that influence adherence and retention to these diet plans. DESIGN: In this preliminary/feasibility study, 48 overweight women will be randomly assigned to one of four diets for 1 year - Atkins (extremely low carbohydrate), Zone (low-carbohydrate, high protein), Ornish (very low fat), and USDA/Food Pyramid (high carbohydrate/moderate-low fat) - and collect data on behavioral and physiological factors. These specific diets have been selected for their wide range of carbohydrate intake (from very-low to very-high carbohydrate: Atkins<Zone<Food-Pyramid<Ornish), and for their widespread use and popularity. IMPLICATIONS: This examination of popular weight loss diets is designed to scientifically test the health claims being made by their proponents. Findings will fill important gaps in our knowledge, and have an impact on both consumer and health professionals who remain confused about the benefits and risks of these diets. The data generated from this preliminary/feasibility study will be used to strengthen the proposal and design of a future full-scale study.